In the field of circuit design, it is well known that when multiple circuits or circuit stages are connected to a common power supply rail, undesired coupling may occur between the circuits or stages. This undesired coupling occurs, for example, when a first circuit switches on, drawing a significant amount of current, and the voltage on the supply rail momentarily drops as a result. Other circuits which receive a supply voltage from that supply rail experience the same drop in supply voltage. In turn this may cause the outputs of such other circuits to fluctuate or otherwise deviate from their normal conditions. For example, a voltage spike or oscillation might appear in the output of another of said circuits. Broadly speaking, the undesired artifact in the latter circuit's output is noise induced by operation of the first circuit. This noise can propagate through yet further circuits or stages and corrupt the device's ultimate outputs or at least degrade its performance.
To reduce or eliminate this noise, it is customary to try to isolate each circuit from noise on the supply voltage rails by providing a capacitor between a common ground and the point where each circuit connects to the supply rail--or as close thereto as possible. These capacitors act as low impedances to high-frequency switching noise and smooth out the voltage at each circuit's supply node.
Unfortunately, in integrated circuit "chips", monolithic capacitors of the size required for effectively decoupling from the supply rails tend to have fairly large capacitance values (e.g., in the range of 22 to 47 microfarads) and require considerable area. Therefore, it is generally not cost-effective to provide within the monolithic structure of an integrated circuit capacitors of sufficient capacitance to provide decoupling to an acceptable level. Additionally, in RISC and DSP processor chips, particularly those of high bandwidth, and other high density digital or analog chips, little room exists on a die for the fabrication of sufficiently large monolithic capacitors. Therefore, either discrete decoupling capacitors are connected to the integrated circuit "chip" within the package that houses the chip (but not within or on the substrate) or they are added externally to the package in a socket or on a printed circuit board or the like, when the system using the packaged chip is assembled. Of course, the further each capacitor is from the associated supply node in the circuit (i.e., the node to be connected to the supply rail), the greater the inductance and resistance between the capacitance and the circuit node. This inductance can interfere with the desired operation of the capacitor, particularly in high-speed circuits.
Power supply decoupling is particularly necessary in high-speed digital circuitry, where transient voltage spikes on power rails can induce erroneous logic level outputs. High-speed processors and memory circuits, for example, may store or provide as output erroneous data if inadequate supply decoupling is provided. However, the provision of in-package decoupling capacitors is complicated with such circuits not only by virtue of the fact that these circuits typically draw large switching currents (such as on the order of 10 Amperes or more), but also because they may dissipate relatively high amounts of power in the form of heat. To facilitate the removal of this heat, a heat-conductive path (which generally is electrically conductive, also) typically is provided between the substrate of the integrated circuitry and the exterior of the package, where it may be placed in contact with a heat sink. Safety considerations and attendant regulations generally require that the heat conductor, which could be touched by maintenance personnel, for example, be maintained at ground potential. In an embedded RAM process, an n-type substrate is often used and an n-type memory area may be placed in a p-well to isolate it from substrate currents generated by high speed digital circuits. Since the substrate is connected to a different supply potential, care is typically taken to isolate the substrate electrically from the heat conductor. Generally this is done at least partly by securing the substrate to the heat conductor with a non-conductive epoxy. However, this introduces a reliability concern for a large die since non-conductive epoxy typically has low shear strength. Alternatives, such as diamond shims are expensive and complicate assembly.
Thus, there is a need for a low cost decoupling capacitor which can be assembled into an integrated circuit package and which presents low inductance. This capacitor preferably will be compatible for use with high-speed digital circuitry, particularly monolithic circuitry which requires a heat conductor affixed to the substrate to conduct excess away therefrom and to an external heat sink which is brought in contact with the package containing the circuitry.